1. Technical Field
The present invention relates generally to memory design evaluation circuits, and more particularly to a memory circuit having a mimicking cell that accurately reflects internal state changes of a memory cell.
2. Description of the Related Art
Storage cell speed, circuit area and environmental operating ranges, e.g., supply voltage and temperature range, are critical limitations in today's processing systems and are predicted to become even more of a critical limitation as technologies move forward. In particular, static random access memory (SRAM) cells are used in processor caches and external storage to provide fast access to data and program instructions. Static storage cells are also used within processors and other digital circuits for storing values internally, for example, in processor registers.
With processor cycle frequencies reaching well above 4 Ghz, development of SRAM cells that can store and provide access to stored values within that period has become necessary. However, actually measuring the internal writeability and write timing margins of memory cells presents a challenge. In a typical storage cell, there is no mechanism for determining writeability and write timing margin, except for empirical evaluation performed by reading the cells after performing write operations under various conditions. If a probe is used to attempt to measure the internal result and timing of a write operation, the probe alters the timing of the cell, yielding incorrect results.
Memory cell transition times involving bitline read operations, sometimes in combination with write operations, have been evaluated using ring oscillator circuits or cascaded cell delay circuits wherein a large number of cells are cascaded. A ring oscillator may be formed with feedback of an output of the last cell to an input of the first cell, or a one-shot delay may be measured through the cascade of cells. The frequency at which the ring oscillator operates or the one-shot delay indicates the transition time performance, which provides some measure of ultimate operating frequency and access times. Typically, the cell design is then changed in subsequent design iterations having parameters adjusted in response to the results of the ring oscillator test.
However, present ring oscillator circuits and other delay-oriented circuits for performing delay tests typically either are not applied on production dies or they do not test the actual storage cells under wordline loading conditions identical to placement of the cells within an array. Further, write cycle measurements are not measured independently, since the inclusion of a cell in the oscillator ring or delay line requires that the cell value will be read in some manner to provide input to the next cell.
It is therefore desirable to provide a test circuit and method for accurately measuring wordline transition times under the complete row loading conditions of an actual array. It would further be desirable to measure access (pass) transistor drive time and internal write state/write timing of a storage cell under actual loading conditions. It is further desirable to provide such a test circuit that can be integrated within a production storage device.